Does anyone have any set methods or systems in place for calculating these percentages? 

The 4% interest rate is an important part of this calculation because the principle paydown portion of your payment is significant, I'm going to guess it's about $9k per year.

 $300k/1945 is 154 months (12.85 years) to have the cash flow recover the additional down payment.  

Likely due to the recent hurricane Michael.we have included coverage for replacement of lost rents for at least 6 months, to cover us for the mortgage payments if a hurricane destroys the property and we have to rebuild.  

There are a lot of local lenders that charge a 1% origination fee and the interest only payments during construction are prime plus .75 - 1.0%.

That said you could ask for a statement showing their recent payments.

I provided an up front $10000 earnest money payment and then secured a monthly interest only loan with principal being paid upon final sale.

Are you looking for lowest payment possible and maybe an interest only or variable rate (but that has higher risk).Once you define what it is you need, then you should start speaking with lenders and ask them what types of products they have for what it is you need.If it is a traditional lender and they are licensed through the nmls (google nmls consumer access to find them), then that means they have been vetted.Note that most lenders DO NOT keep their loans and they get sold, so age of company is usually not important, experience of originator would be more priority for me

This deposit serves as a form of protection for any potential damages or outstanding rent at the end of the lease term.Most of the property management companies offer various convenient payment methods to ensure a hassle-free experience for tenants.

I want to use the money from that sale to pay for the renovation or as a down payment for my father’s house in Pasadena (if I decide the purchase the house).